1. Field of the Invention
This invention relates to the continuous production of fabric reinforced cementitious panel webs suitable for cutting into individual reinforced panels used as backerboards for ceramic tile, and other facing materials. Such reinforced panels are of the type generally described in U.S. Pat. Nos. 3,284,980, 4,159,361 and 4,450,022.
Such panels typically comprise a core layer consisting of a hydraulic cement, such as portland cement, mixed with a light weight aggregate and/or a foaming agent, and a web of reinforcing fabric bonded to each face of the core layer. The panels are described in detail in the Dinkel Pat. No. 3,284,980.
Production of such fabric reinforced panels, and of the fabric reinforced cementitious panel webs from which the panels are cut, is carried out commercially by two similar processes. The first method comprises sequentially laying down on a conveyor belt, carrier means such as a continuous web of plastic coated paper or individual plastic sheets, a web of pervious reinforcing fabric, such as a web of woven glass fiber mesh to which a cementitious slurry has been applied, a layer of cementitious core mix and a second web of pervious reinforcing fabric to which a cementitious slurry has been applied, cutting the uncured ribbon (i.e. panel web) so formed and stacking for curing. In the second method the web of pervious reinforcing fabric without any slurry is laid down on a continuous web of plastic coated paper, a layer of cementitious core mix is then laid on the fabric layer, followed by a second web of bare pervious reinforcing fabric laid down on top of the core, the layers being united as by vibration. In this method the panel web so formed is then cured at least to a hardened condition followed by cutting into individual panels.
Major problems are encountered in the manufacture of such reinforced cementitious panel webs, in the handling and placing of the web of reinforcing fabric on the core layer, and in the application of a proper amount of the cementitious material to the reinforcing web to obtain effective bonding of the web layers to the core layer. In the manufacturing process where only a cementitious core mix is employed (and no slurry is applied to the mesh web) it is difficult to obtain penetration of the core mix through the openings in the mesh to permit good bonding of the mesh to the core.
In the present known methods of manufacture of such panels, the webs of reinforcing fabric, as well as the wet uncured ribbon of cementitious panel material, are drawn through the forming operations under tension. The webs of fabric, as they are drawn into the forming operation, tend to neck-in, due to the tension applied. In the one method the web of fabric is fed into the operation in a width equal to the final width of the panel web being produced. The necking-in reduces the width of the fabric web to less than that of the panel web. The result is that an area along one edge of the panel web, or along both edges, is left devoid of reinforcement; the core material in this area is left with no surface layer. Such edges have little or no integrity and are subject to crumbling or breaking, even where only one of the two reinforcing webs necks-in to less than the panel width.
In order to avoid such defects, in one commercial process the cementitious panel web is made over-size in width and the excess is then trimmed off the edges, usually after the cementitious panel web has hardened sufficiently to facilitate cutting. In this type of process necking-in of the web does not impair the edges of the product but this method adds substantially to the cost of the panels. The fabric reinforcement is, by far, the most expensive component used in production of such cementitious panels and since both edges must be trimmed to produce the final width of panel, a sizeable expense results. There is a double waste since the reinforcing fabric is over-size both on the top and the bottom. In addition, there is the waste of core material cut off and the labor cost of carrying out the trimming operation, as well as the cost of disposal of the trimmings.
The tension applied to the uncured panel web in order to tow it or draw it along the forming line tends to cause the top layer of reinforcing fabric to slip back whenever the conveyor belt is stopped and re-started. This is particularly likely to occur where the panel web is cut immediately after it is formed and before curing. In one of the conventional production methods the conveyor belt is stopped momentarily as a panel length is cut and the belt is then re-started. As the conveyor belt again moves forward the top layer of reinforcing fabric tends to pull back from the cut end; this is due to the pull on the fabric acting counter to the tension.
Displacement of the web, particularly the top web, as by pulling in from the edges, occurs even when the individual panels are severed by a flying cutter and the flow of the panel web is not interrupted. The tension applied is not always uniform as the ribbon (panel web) is towed along through the production line, and the reinforcing fabric is readily displaced because of the wet slurry that the fabric rides in.
Another serious problem in prior methods is that they are lacking in means for compensating for differences in the viscosity of the slurry, and also of the core material, during running of the forming machine. This problem is most evident where a layer of fabric bathed with slurry is deposited on top of the core layer. The amount of slurry that can be picked up by a given mesh web drawn through a slurry bath is a function of the viscosity of the slurry. Where the slurry viscosity is low only a limited amount of slurry is carried by the mesh, with the result that the web of mesh is only lightly bonded to the core. This is particularly true in the case of the layer of mesh that is laid on top of the core; the slurry carried on the mesh tends to flow by gravity into the core, thereby starving the mesh. The result is a serious defect of inadequately bonded mesh and potential delamination. Where the viscosity of the slurry is too high the web of mesh will pick up an excess of slurry as it passes through a bath. The result will be a thick, hard surface on the cured panel increasing the weight of the panel undesirably and impairing the nailability of the panel.
The third major problem is that of securing penetration of the core mix through the openings of the reinforcing fabric when bare fabric (with no cementitious slurry applied) is employed. The single composition core mix employed is highly viscous and penetrates the openings in the mesh only with difficulty. Galer, in U.S. Pat. No. 4,450,022, points out that inadequate penetration of the openings in the reinforcing mesh is common to all methods of production of mesh reinforced cementitious panels. In order to overcome this problem it has been necessary to use a mesh with larger openings and a lower yarn count per inch, such as 8.times.8 or 10.times.10, resulting in a panel of lower flexural strength. Another expedient offered to improve penetration of the bottom mesh is that of a step-down in the conveyor table which allows the mesh with core to be displaced from the carrier sheet. This requires added equipment and still requires the mesh with large openings.
The term "cementitious" as used herein refers to any composition containing a hydraulic cement such as portland cement. The term "slurry" refers to a flowable mixture of water and a hydraulic cement. The term "cementitious core mix" (and alternately "core mix") refers to a mixture of a hydraulic cement, water and aggregate such as sand, expanded shale or clay, expanded polystyrene beads, slag and similar materials, as well as foaming agents, modifiers and the like. The term "pervious reinforcing fabric" (and alternatively "fabric") refers to a layer or mat of fibers suitable for use in concrete, having openings sufficiently large to permit penetration of the cementitious slurry or core mix into and through the openings so as to permit adequate bonding of the fabric to the core; the most commonly used such fabric is a woven heat-set mesh of vinyl coated glass fiber yarns. The term "cementitious composition" as used herein refers collectively to cementitious slurries and cementitious core mixes.
2. Description of the Prior Art
In U.S. Pat. No. 3,284,980 (Dinkel) a cementitious panel having a core of lightweight aggregate and portland cement with a layer of a woven mesh of glass fiber yarns bonded to each of the two major faces by means of a relatively high density layer of hydraulic cement, is described. The panels are shown as being formed in molds, with the slurry of hydraulic cement (portland cement) being applied to the layer of glass fiber mesh in the mold by means of a traveling supply pipe. The slurry penetrates the openings in the fabric and fills them so that the fabric layer is enveloped on both sides with the slurry. The patent is not concerned with a continuous production process nor with tension on the fabric.
U.S. Pat. No. 3,509,010 (Metzger) describes a method of producing a cementitious building panel by impregnating a strip of covering material (e.g. glass fiber sheet) with a hydraulic cement slurry, depositing a layer of expanded clay particles and hydraulic cement on the cover material to form a composite continuous strip, hardening the continuous strip and then cutting the continuous strips into panels (building components). This patent is silent as to how the glass fiber sheet is impregnated with the hydraulic cement slurry.
U.S. Pat. No. 4,159,361 (Schupack) describes a flexible reinforced cementitious panel which is cold deformable, suitable for bending to form drums, culverts, pipes and the like. Production of the panels is shown as being carried out by moving a train back and forth across a forming table whereby individual panels are formed one on top of the other. The train lays down a length of reinforcing fabric such as coated fiberglass, then a layer of cementitious composition which is smoothed by an oscillating screed and then lays down a second layer of fabric. The entire assembly is vibrated to encapsulate the reinforcing fabrics in the cementitious composition.
U.S. Pat. No. 4,203,788 (Clear) describes a method and apparatus for producing the Dinkel cementitious panel. In the described process the web of reinforcing fabric, namely fiberglass mesh, is impregnated with a cementitious slurry by drawing it through a bath of the slurry to fill the voids in the mesh and to accumulate slurry on both sides of the mesh. In the next step the excess slurry is doctored off the mesh, followed by a dragging step.
The mesh is then laid on carrier sheets, a layer of cementitious core composition is deposited on the mesh and a second layer of slurry impregnated mesh is laid upon the core. A panel web is thereby formed which is then cut into panel lengths. This process has a serious disadvantage in that the webs of reinforcing fabric are under tension until after the cutting step; necking-in of the fabric and defective panel edges frequently occur. Also, this process is seriously deficient in another respect. While suitable for securing bonding of the bottom layer of mesh to the core, it does not permit controlled bonding of the top layer of mesh to the core in that it does not have the means for adjusting operations to compensate for variations in the viscosity of the cementitious slurry. The amount of slurry picked up by the mesh as it is drawn through the slurry bath is entirely dependent upon the slurry viscosity.
In U.S. Pat. No. 4,298,413 (Teare) the cementitious slurry is applied to the glass fiver mesh by a series of transfer rollers. This method is limited in that the transfer of the desired thickness of slurry film from one roller to another is difficult to achieve with a portland cement slurry; the film thickness varies excessively.
In U.S. Pat. No. 4,450,022 Galer describes a method of producing fabric reinforced cementitious panels in which no slurry or other bonding agent is separately applied to the web of reinforcing mesh. Instead, a first web of mesh is laid on a web of carrier paper supported on a conveyor belt, an aqueous cementitious mixture is discharged onto, and spread across, the mesh to a desired thickness, and a second web of mesh is laid on top of the layer of cementitious mix and pushed into the mix by a screed projecting just below the surface, combined with a vibration action. This process has the disadvantage of inadequate penetration of the mesh openings by the core mix unless a large opening mesh is employed as well as vibration and a conveyor belt step off.
In U.S. Pat. No. 4,477,300 (Pilgrim) a cementitious board is formed by depositing a heavy slurry of gypsum plaster between two webs of paper board or glass fiber tissue and pressing the two webs toward each other to achieve the desired thickness. Vibration is applied to the webs to cause unwanted air bubbles trapped in the slurry to be removed.
U.S. Pat. No. 4,504,335 (Galer) describes a method of producing fabric reinforced cementitious panels which is similar to that described in U.S. Pat. Nos. 4,488,917 and 4,450,022. In U.S. Pat. No. 4,504,335 the top web of fabric is pressed into the layer of core material and the final thickness of the core layer (mortar) is regulated by a reversely turning roller instead of by a screed blade or trowel. This process requires the use of tension in that the reinforcing fabric (network) must be dragged through the slit between the roller and supporting surface (conveyor belt).